Microscopic primordial black holes as macroscopic dark matter from large extra dimensions

TL;DR

This paper explores how primordial black holes in models with large extra dimensions can grow significantly in the early universe, potentially becoming the dark matter without requiring large initial formation fractions.

Contribution

It demonstrates that extra dimensions enable microscopic black holes to grow to macroscopic sizes, lowering the initial abundance needed for dark matter.

Findings

Black holes can grow by many orders of magnitude due to accretion in extra-dimensional models.

The critical initial abundance for dark matter is dramatically reduced in these scenarios.

PBHs can reach solar-mass scales by matter-radiation equality in the early universe.

Abstract

We study the coupled cosmological evolution of primordial black holes (PBHs) and radiation in the Arkani-Hamed-Dimopoulos-Dvali (ADD) framework with $n$ large extra dimensions and a fundamental gravity scale $M_\star$ at the TeV scale. For PBHs with horizon radius smaller than the compactification scale, the higher-dimensional geometry implies a larger horizon size at fixed mass and therefore a suppressed Hawking temperature. As a result, radiation accretion can overcome evaporation in the early Universe and drive a ``runaway'' phase of rapid mass growth. By numerically solving the coupled mass and energy-density evolution equations, we show that for $n \geq 2$ initially microscopic PBHs with initial mass $M_i \gtrsim 10^{12}\,$g can grow by many orders of magnitude and potentially reach macroscopic, even solar-mass, scales by matter-radiation equality. We determine the critical initial abundance $\beta_{\rm crit}$ required for PBHs to account for the observed dark matter density and find that extra dimensions dramatically lower this threshold, allowing viable scenarios with $\beta_{\rm crit}\sim 10^{-44}$. This identifies a previously unexplored region of parameter space in which the dark matter abundance is achieved through dynamical mass growth rather than large initial collapse fractions.